Practical and Stereoselective Synthesis of a Pentacyclic Guanidine System: Synthetic Studies toward Ptilomycalin A and Related Compounds

Symmetrical pentacyclic guanidine compounds 3a–c have been synthesized based on the construction of 2,5-disubstituted pyrrolidines via sequential 1,3-dipolar cycloaddition and the formation of pentacyclic guanidine via guanylation followed by double N,O-acetalization. The present synthesis will provide a potential route for the synthesis towards ptilomycalin A (1) and 13,14,15-isocrambescidin 800 (2).